1. Technical Field of the Invention
The present invention relates generally to semiconductor devices and, more particularly, to a concentrated photovoltaic (CPV) receiver package or module which is assembled to include a leadframe assembly comprising multiple frames stacked on top of each other in to provide high thermal dissipation and high voltage isolation, while at the same providing a high level of reliability with a comparatively low manufacturing cost.
2. Description of the Related Art
Photovoltaic cells or dies are a well known means for producing electrical current from electromagnetic radiation. Traditional photovoltaic cells comprise junction diodes fabricated from appropriately doped semiconductor materials. Such devices are typically fabricated as thin, flat wafers with the junction formed parallel to and near one of the flat surfaces. Photovoltaic cells are intended to be illuminated through their so-called “front” surface. Electromagnetic radiation absorbed by the semiconductor produces electron-hole pairs in the semiconductor. These electron-hole pairs may be separated by the electric field of the junction, thereby producing a photocurrent. Currently known photovoltaic cells typically have a generally quadrangular (e.g., square) configuration defining four peripheral side edges, and include a pair of bus bars which are disposed on the top or front surface and extend along respective ones of an opposed pair of the side edges. The bus bars are used to facilitate the electrical connection of the photovoltaic cell to another structure, as described in more detail below.
There is currently known in the electrical arts semiconductor devices known as CPV receiver die packages or modules. Currently known CPV modules typically comprise a ceramic substrate having a conductive pattern disposed on one side or face thereof. Attached to the substrate and electrically connected to the conductive pattern are electrical components, including a pair of preformed wire connectors and a packaged diode. Also attached to the substrate and electrically connected to the conductive pattern thereof is a CPV receiver cell or die. The electrical connection between the receiver die and the conductive pattern is often facilitated by a pair of punched thin metal foil or braided ribbon/mesh connectors which extend along and are welded or soldered to respective ones of opposed sides of the receiver die, which typically has a quadrangular or square configuration as indicated above. More particularly, the pair of punched thin metal foils or braided ribbon/mesh connectors are welded or soldered to respective ones of the bus bars on the top or front surface of the receiver die. In certain existing CPV modules, the electrical connection of the receiver die to the conductive pattern is facilitated by the use of multiple wires bonded to the bus bars on the front surface of the receiver die and the bond pads of the conductive pattern of the substrate, the wires being used as an alternative to the aforementioned braided ribbon or mesh interconnects. These wire bonds are often fabricated from gold, and are provided in differing numbers and/or diameters depending on the design of the CPV module. The CPV module may further include a light concentration means which is adapted to concentrate solar radiation onto the front surface of the receiver die.
In certain currently known CPV receiver die packages or modules, anywhere from about 500 to about 1500 suns of energy will typically be focused on the receiver cell or die of the CPV package, such receiver die converting the impinging light into electricity and heat which must be effectively dissipated from the CPV package as well. In this regard, the total incoming energy often falls within the range of from about 45 to about 225 watts, of which about 65% is typically converted to heat that must be managed in the CPV package. CPV packages are often electrically connected to each other in series within panels, with multiple panels also being connected in series. These systems often operate at very high voltage, with the requirement typically being that each CPV package provides about 5,000 volts of electrical isolation. The requirements for high thermal dissipation as well as high voltage isolation, coupled with market demands for low cost as well as high reliability, are not satisfied by currently known CPV receiver die packages or modules.
In an attempt to satisfy the aforementioned desired attributes of high thermal dissipation coupled with high voltage isolation in a CPV package, one of the leading solutions provided in the CPV industry is the use of a ceramic core substrate with metal conductors adhered to one or both of its top and bottom surfaces. The ceramic material provides thermal conductivity from the top metal to the bottom metal, as well as voltage isolation between the circuit metallization on the top surface and the external environment. Another approach is to use more traditional semiconductor leadframe based solutions. However, the attempted leadframe solutions have generally been unsuccessful since providing good thermal conductivity concurrently with required voltage isolation has increased the complexity of these solutions, and hence the cost. Even the use of the aforementioned ceramic core substrate with metal conductors as a solution has proven to present difficulties in finding a good tradeoff between high thermal performance, electrical isolation and cost.
The present invention provides a matrix leadframe based approach in a unique assembly sequence to the addresses these and other shortcomings of the prior art CPV packages. In the CPV package of the present invention, a leadframe assembly is provide, such leadframe assembly including multiple frames stacked on top of each other. A top frame provides the electrical interconnect between the top or front surface of the receiver die and the bypass diode required to complete the circuit. The top frame also provides the attach pad or connector for a hook up wire which carriers the current away from the CPV package. In the assembly of the CPV package of the present invention, the top frame is placed into an empty fixture and solder paste is screen printed on areas which will connect to the receiver die, the diode, and a stand off die. The standoff die is a mechanical component only, and serves to maintain the separation between the top frame hook up wire pad and a middle frame of the assembly. The receiver die, the diode, and the standoff die are then each placed onto the top frame in a flip chip orientation so that the bus bars on the receiver die, the anode of the diode and a metalized surface of the standoff die are each in contact with corresponding connection points on the top frame. The assembly is then put through a reflow oven to join the receiver die, diode, and standoff die to the top frame.
In the next process step, the aforementioned middle frame is placed into an empty fixture and run through a dispensing process which deposits a solder paste under where the receiver die and diode will be connected thereto. Reusable shims are placed over locating features in the fixture and on top of side rails of the middle frame. These shims are used to maintain a prescribed gap between the top and middle frames. The top frame is then placed onto the fixture using features in the fixture to ensure the required alignment between the top and middle frames. The assembly is then put through a reflow. After the top and middle frames have been joined, the handling rails and interconnect structure (i.e., the support structure) of the top frame are cut away so that only the hook up wire pads will electrically accessible outside of the final CPV package. However, the middle frame retains its handling rails and interconnect structure.
In the next process step, a bottom frame is placed into an empty fixture and run through a dispensing process during which a thermally conductive, but electrically isolating thermal interface material (or TIM material) is deposited thereon. The previously joined top and middle frames are placed into the fixture and located relative to the bottom frame using features in the fixture. The previously deposited TIM material makes contact between the middle and bottom frames. The assembly is then placed into an oven and cured. After cure, the handling rails and interconnect structure of the middle frame are cut away such that only the hook up wire pads of the top frame will be electrically accessible outside of the completed CPV package. The bottom frame retains its handling rails and interconnect structure. The three layer structure including the top, middle and bottom frames is then over-molded such that the top of the receiver die, the two hook up wire interconnect pads and the bottom surface of the bottom frame remain exposed. Finally, the bottom frame handling rails and interconnect structure are then cut away leaving the finished and singulated CPV package.
Part of the uniqueness of the above-described CPV package lies in connecting a leadframe to the bus bars of the receiver die and to the bypass diode using solder, which provides the CPV package with much lower electrical resistance, while also improving the power out of the CPV package. Further, connecting the bottom of the receiver die and the cathode of the bypass diode using solder provides the CPV package with much lower electrical resistance, and improves the power out of the package as well. Still further, the TIM material used in the CPV package of the present invention provides lower thermal resistance, and is thinner then the alumina structures currently in use of existing CPV packages, thus providing superior thermal management. Further, that the top and middle frames of the CPV package have no exposed metal other than the hook up wire interconnect pads makes the CPV package inherently superior from a voltage isolation standpoint. The exposed heat spreader defined by the exposed bottom surface of the bottom frame also provides a superior interface to the next level of thermal management. Thus, the CPV package of the present invention provides superior electrical and thermal performance, while providing approximately 5,000 volts of isolation demanded by most applications. These and other features of the present invention will be described in more detail below.
Common reference numerals are used throughout the drawings and detailed description to indicate like elements.